This invention relates generally to chemical sensing and more particularly to the sensing of changes in concentration of a selected chemical, such as CO2, using non-linear sensors to detect changes in attenuation of an optical beam being transmitted through a medium potentially containing some of a such selected chemical, such as infrared sensors.
Absolute measurements and accuracy are generally of interest in many chemical sensing applications. However, in certain applications the change (delta) that occurs in the chemical concentration during a process is of more interest than the absolute value. Among the chemical sensor technologies currently available, some of them are inherently linear in nature (e.g., electrochemical) and some are non-linear (e.g., infrared and metal oxide). In the non-linear sensors, the non-linear measured parameter is converted linearly using appropriate electronics/software. For example, in infrared sensors, the measured parameter, infrared absorption, increases as in FIG. 1 with concentration. Knowing this curve shape, instruments are designed to linearize the output, mostly by software. The curve information is stored in the sensor microprocessor during calibration and used during the linearization routine.
Delta measurements are directly related to the slope of the measured parameter vs. concentration curve (example shown in FIG. 1). For small changes in concentration,
Change in Concentration (Delta)=Slopexc3x97Change in the measured absorption parameter.
In non-linear sensors, since the slope changes with the concentration, the delta measurement will be accurate only if the absolute concentration is known accurately. Normally, chemical sensors often tend to drift over time and need to be calibrated periodically to maintain accuracy. In some applications (for example, residential applications), re-calibration of a chemical sensor is not very feasible. In a drifted sensor, the absolute concentration measured and hence the slope used at that concentration will not be accurate. This will cause the delta measurement also to be inaccurate. The more the sensor drifts, the more the inaccuracy in delta measurement. So in situations where calibration is not feasible, the error in the delta measurement can become unacceptably high.
In gas furnace heat exchanger leak detection for example, the CO2 level is measured in the air-side of the furnace just before and after the furnace is fired. If the furnace heat exchanger has a leak, the CO2 level will increase significantly. A threshold is normally set on this delta CO2 value to set an alarm and shut off the furnace. The delta measurement will be accurate only if the sensor is calibrated and holds its calibration during the measurement. Since gas sensors are normally prone to some degree of drifting and hence the delta measurement, over time, will not be accurate. On the other hand, furnace manufacturers expect the leak detection to last for 10 years or more and re-calibration is not a practical alternative.
It is an object of the present invention to provide a method for measuring the change in concentration of a selected chemical, such as a selected gas, which measurement has reduced error caused by calibration changes in the chemical sensor over time. Another object is the provision of a method for measuring a change in concentration of CO2 in a gas furnace leak detection application which has improved accuracy over an expected life without re-calibration of the sensor used in performing the measurements. Yet another object of the invention is to overcome the above noted prior art limitations.
Briefly described, a method for sensing the change in concentration of a selected chemical, such as the gas CO2, according to the invention comprises the steps of measuring a first infrared signal Ig(1) by means of a suitable gas sensor such as an NDIR (non-dispersive infrared sensor), using the relation: absorption=1xe2x88x92Ig/Io where Ig is the infrared signal and Io is the zero gas, or base line, signal, using the absorption value under a preselected gas concentration C(1) calculating the corresponding Io(1) value, then measuring a second infrared signal Ig(2), calculating a new absorption value using the calculated Io(1) value for Io and respective concentration C(2) and subtracting the first concentration C(1) from the second concentration C(2) to obtain the change in concentration. By choosing the preselected gas concentration C(1) to be intermediate to two extremes, the divergence of a drifted sensor can be limited to an acceptable level.